refactor: move capacity_forecast_service from billing to monitoring

Resolves the billing (core) → monitoring (optional) architecture violation
by moving CapacityForecastService to the monitoring module where it belongs.

- Create BillingMetricsProvider to expose subscription counts via stats_aggregator
- Move CapacitySnapshot model from billing to monitoring
- Replace direct MerchantSubscription queries with stats_aggregator calls
- Fix middleware test mocks to cover StoreDomain/MerchantDomain fallback chains

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/monitoring/models/__init__.py

@@ -2,11 +2,10 @@
 """
 Monitoring module database models.
 
-Re-exports monitoring-related models from their source locations.
+Provides monitoring-related models including capacity snapshots.
 """
 
-# CapacitySnapshot is in billing module (tracks system capacity over time)
-from app.modules.billing.models import CapacitySnapshot
+from app.modules.monitoring.models.capacity_snapshot import CapacitySnapshot
 
 # Admin notification and logging models
 from app.modules.messaging.models import AdminNotification

app/modules/monitoring/models/capacity_snapshot.py

@@ -0,0 +1,71 @@
+# app/modules/monitoring/models/capacity_snapshot.py
+"""
+Capacity snapshot model for platform capacity monitoring.
+
+Stores daily snapshots of platform metrics for growth trending and capacity forecasting.
+"""
+
+from sqlalchemy import (
+    Column,
+    DateTime,
+    Index,
+    Integer,
+    Numeric,
+)
+from sqlalchemy.dialects.sqlite import JSON
+
+from app.core.database import Base
+from models.database.base import TimestampMixin
+
+
+class CapacitySnapshot(Base, TimestampMixin):
+    """
+    Daily snapshot of platform capacity metrics.
+
+    Used for growth trending and capacity forecasting.
+    Captured daily by background job.
+    """
+
+    __tablename__ = "capacity_snapshots"
+
+    id = Column(Integer, primary_key=True, index=True)
+    snapshot_date = Column(DateTime(timezone=True), nullable=False, unique=True, index=True)
+
+    # Store metrics
+    total_stores = Column(Integer, default=0, nullable=False)
+    active_stores = Column(Integer, default=0, nullable=False)
+    trial_stores = Column(Integer, default=0, nullable=False)
+
+    # Subscription metrics
+    total_subscriptions = Column(Integer, default=0, nullable=False)
+    active_subscriptions = Column(Integer, default=0, nullable=False)
+
+    # Resource metrics
+    total_products = Column(Integer, default=0, nullable=False)
+    total_orders_month = Column(Integer, default=0, nullable=False)
+    total_team_members = Column(Integer, default=0, nullable=False)
+
+    # Storage metrics
+    storage_used_gb = Column(Numeric(10, 2), default=0, nullable=False)
+    db_size_mb = Column(Numeric(10, 2), default=0, nullable=False)
+
+    # Capacity metrics (theoretical limits from subscriptions)
+    theoretical_products_limit = Column(Integer, nullable=True)
+    theoretical_orders_limit = Column(Integer, nullable=True)
+    theoretical_team_limit = Column(Integer, nullable=True)
+
+    # Tier distribution (JSON: {"essential": 10, "professional": 5, ...})
+    tier_distribution = Column(JSON, nullable=True)
+
+    # Performance metrics
+    avg_response_ms = Column(Integer, nullable=True)
+    peak_cpu_percent = Column(Numeric(5, 2), nullable=True)
+    peak_memory_percent = Column(Numeric(5, 2), nullable=True)
+
+    # Indexes
+    __table_args__ = (
+        Index("ix_capacity_snapshots_date", "snapshot_date"),
+    )
+
+    def __repr__(self) -> str:
+        return f"<CapacitySnapshot(date={self.snapshot_date}, stores={self.total_stores})>"